Pescadero Basin Hydrocarbons

Occasionally I will read an article and it will take me on “an unexpected journey.”  I ran across this very interesting article on Real Clear Science today:

July 24, 2017

New study challenges prevailing theory about how deep-sea vents are colonized

An article just published in the Proceedings of the Royal Society B describes two remarkably different hydrothermal vent fields discovered in the southern Gulf of California. Despite being relatively close together, these vents host very different animal communities. This finding contradicts a common scientific assumption that neighboring vents will share similar animal communities. Instead, the new paper suggests that local geology and the chemistry of the vent fluids are important factors affecting vent communities.


Though neighbors, the Alarcón Rise and Pescadero Basin vent fields are geologically very different. The seafloor along the Alarcón Rise is covered in young, fresh lava, and the fluids spewing out of the vents are very hot (up to 360 degrees Celsius) and rich in metal sulfides that form dark, crumbly chimneys known as “black smokers.” Animals at the Alarcón Rise are similar to locations further south (almost 300 kilometers) on the East Pacific Rise.

In Pescadero Basin, however, hydrothermal-vent fluids pass through thick layers of seafloor mud. As the hot hydrothermal fluid flows through this mud, it “cooks” organic material, forming methane (natural gas) and oil-like hydrocarbons. The Pescadero Basin vents contain very little sulfide, and the superheated fluids produce giant, light-colored, carbonate chimneys streaked with dark, oily hydrocarbons.



However, I quickly lost interest in the biological aspects of the article when I read this bit:

In Pescadero Basin, however, hydrothermal-vent fluids pass through thick layers of seafloor mud. As the hot hydrothermal fluid flows through this mud, it “cooks” organic material, forming methane (natural gas) and oil-like hydrocarbons. The Pescadero Basin vents contain very little sulfide, and the superheated fluids produce giant, light-colored, carbonate chimneys streaked with dark, oily hydrocarbons.

Could this be actual evidence of “abiotic” or “abiogenic” oil?

No.  Of course it isn’t.  If anything this might just be a “nail in the coffin” for abiotic oil.

The “oil-like hydrocarbons” were associated with hydrothermal-vent fluids which “pass through thick layers of seafloor mud”  in the Pescadero Basin.

Just 75 km to the south, the seafloor of the Alarcón Rise is covered with layers of relatively fresh lava flows and very little sediment.  The Alarcón Rise hydrothermal vents are run of the mill black smokers, with no evidence of “hydrothermal oil.”



This map shows the major known hydrothermal vent sites in and around the Gulf of California. Even though the Pescadero Basin and Alarcón Rise are relatively close, they host very different animal communities. Base map: Google Earth

Here’s an older article about the initial discovery of the Pescadero Basin hydrothermal vents:

June 2, 2015

MBARI researchers discover deepest known high-temperature hydrothermal vents in Pacific Ocean

In spring 2015, MBARI researchers discovered a large, previously unknown field of hydrothermal vents in the Gulf of California, about 150 kilometers (100 miles) east of La Paz, Mexico. Lying more than 3,800 meters (12,500 feet) below the surface, the Pescadero Basin vents are the deepest high-temperature hydrothermal vents ever observed in or around the Pacific Ocean. They are also the only vents in the Pacific known to emit superheated fluids rich in both carbonate minerals and hydrocarbons. The vents have been colonized by dense communities of tubeworms and other animals unlike any other known vent communities in the in the eastern Pacific.

Reflecting on the discovery, Clague commented, “Before the AUV survey of Pescadero Basin, all we knew was that this area was really deep and filled with sediment. I was hoping to find a few outcrops of lava on the seafloor. But we got lucky. The vent field was right on the edge of our survey area, along a fault at the western edge of the basin.”


The AUV and ROV dives showed that the new field extends for at least 400 meters (one quarter mile) along this fault. Within this area the researchers found at least three active hydrothermal chimneys up to 12 meters (40 feet) tall, as well as dozens of low mounds that are most likely collapsed chimneys.

After his ROV dive, Clague noted, “This site was not at all what I was expecting.” For one thing, the fragments of chimneys that the ROV brought back to the surface were quite different from those collected at other vents in the area. The Pescadero chimneys consisted entirely of light-colored carbonate minerals instead of the dark sulfide minerals that are abundant in hydrothermal chimneys elsewhere in the Gulf.

The Pescadero Basin is only the second place in the world where carbonate chimneys (instead of ones made primarily of sulfides) have been found in the deep sea. The other known location is the “Lost City” vent field in the middle of the Atlantic Ocean, at a spot on the Mid-Atlantic Ridge.

The geologists also noticed that their rock samples smelled like diesel. They hypothesize that hot hydrothermal fluids migrating upward through the thick sediments of the Pescadero Basin “cook” organic matter in the sediment, converting it into petroleum-like hydrocarbons—a process that has been observed at several other vents in the Pacific. Hydrocarbons may provide nutrition for the unusual microbes that thrive at these vents.



This is a map of total sediment thickness for most of the world’s oceans from NOAA:


Whittaker, Joanne, Alexey Goncharov, Simon Williams, R. Dietmar Müller, German Leitchenkov (2013) Global sediment thickness dataset updated for the Australian-Antarctic Southern Ocean, Geochemistry, Geophysics, Geosystems. DOI: 10.1002/ggge.20181

I enlarged the area comparable to the MBARI map and posted the four vent systems on it.  Note that the basins with thick sediments exhibit evidence of petroleum-like substances associated with hydrothermal vents.  The vents associated with the sediment-deprived rises do not.

Hydrothermal oil

Petroleum-like substances have been associated with hydrothermal vents in basins with thick organic-rich sediments. However nearby hydrothermal vents with little to no sediment cover (rises) do not exhibit evidence of “hydrothermal oil.”

Furthermore, the “hydrothermal oil” of the Guaymas Basin is extremely young…

Nature 342, 65 – 69 (02 November 1989); doi:10.1038/342065a0

Hydrothermal oil of Guaymas Basin and implications for petroleum formation mechanisms


*Refineria de Petroleo Concon, Casilla 242, Concon, Chile
†Petroleum Research Group, College of Oceanography, Oregon State University, Corvallis, Oregon 97331, USA

PETROLEUM-LIKE hydrocarbons have been detected in thermally altered Recent sediments of Guaymas Basin1–5 and petroleum-like hydrocarbon impregnations were found in hydrothermal mounds on the sea floor and associated with hydrothermal vent emissions5–9. Here we report the evaluation of such a hydrothermal oil, which we find to be similar to conventionally exploited crude oils. Its young geological age (< 5,000 yr, 14C) 10 indicates that a significant fraction of the organic carbon in the oil has completed the transformation from biomass to migrating oil in less than 5,000 years, thus limiting the oil generation, explusion and migration processes to a geologically short timescale. We estimate the generation potential of such hydrothermal oil and discuss its implications to our understanding of the petroleum generation, expulsion and migration mechanisms.

Oil formed in the mantle would be just a bit older than 5,000 years.  The Lost City hydrothermal vent on the Mid-Atlantic Ridge and the Pescadero Basin are the only two known places where carbonate (rather than sulfide) chimneys have been found.  Lost City is also possibly an example of the Fischer-Tropsch process is at work…


Deep-ocean vents are a source of oil and gas

Hydrocarbons bubble up from the mid-Atlantic’s Lost City.

Rachel Courtland

Undersea thermal vents can yield unexpected bounty: natural gas and the building blocks of oil products. In a new analysis of Lost City, a hydrothermal field in the mid-Atlantic, researchers have found that these organic molecules are being created through inorganic processes, rather than the more typical decomposition of once-living material.

Most of the planet’s oil and natural gas deposits were created when decomposing biological matter is ‘cooked’ in high temperatures underground. But non-biological hydrocarbons have also been found deep inside the Earth, where chemical processes create the molecules from inorganic sources such as rock.


Among other measurements, the team analysed the amount of carbon-13 in methane, which contains one carbon atom, and in hydrocarbons containing two, three, and four carbon atoms. As the number of carbon atoms rose, the concentration of carbon-13 fell — the opposite trend to that seen in biologically derived hydrocarbons.

Instead, the pattern of isotopes suggest that a chemical process called the Fischer-Tropsch process is at work in Lost City, creating bigger and bigger hydrocarbons in the hydrogen-rich environment. Although the concentrations were too low to detect without a filter, small amounts of larger hydrocarbons such as kerosene and octane may also be produced.

The team also found that the methane in Lost City contained no carbon-14, suggesting the carbon source for the hydrocarbons comes from within the mantle, far away from organisms that might have had contact with the global carbon cycle at the surface.



Setting aside the fact that “the building blocks of oil products” are not the same thing as oil; in much the same manner that a 2×4 is not the same thing as a house… The carbon in the Lost City hydrocarbons is either so old that carbon-14 is undetectable or it has never “had contact with the global carbon cycle at the surface.”



One Response to “Pescadero Basin Hydrocarbons”

  1. chaamjamal Says:

    Interesting post

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